1. Technical Field
The present invention relates to an illumination device, such as a backlight unit, for use in, for example, a liquid crystal display of a mobile telephone, and to a light guide plate in the illumination device.
2. Related Art
In liquid crystal display devices, a backlight unit is provided on the back side of a liquid crystal display panel to perform transmissive display. In general, the backlight unit is constructed as an illumination device including a light source, a light guide plate for applying light emitted from the light source as planar light onto the back surface of the liquid crystal display panel, a sheet for diffusing the light from the light guide plate, and a prism sheet for collecting the light. When light from the light source enters the light guide plate, it is repetitively reflected between an emergent surface and a reflecting surface of the light guide plate, and finally emerges outside from the emergent surface.
When the light guide plate is a flat plate having a fixed thickness and the emergent surface is a mirror-finished surface, light does not emerge from the emergent surface. Accordingly, JP-T-2000-504855 discloses a light guide plate in which the angle of light is gradually changed every time the light is reflected by a prism-shaped pattern provided on a reflecting surface, and the light finally emerges from an emergent surface when the angle with respect to the emergent surface exceeds the critical angle.
In the above light guide plate, however, when a point light source, such as an LED, is used, luminance nonuniformity is caused because light diffusibility is insufficient. For this reason, the light utilizing efficiency of the light guide plate is not high, and the light-emitting area of the emergent surface is limited.